In spread-spectrum radio communication systems, the reception quality is improved by separating or combining a plurality of paths identified by a reception device. When a transmission device and a reception device use a plurality of antennas, the reception quality can be improved by selecting and combining paths using the same technique.
With reference to FIG. 1, a spread-spectrum radio communication system disclosed in Japanese Unexamined Patent Application Publication No. 2001-230702 is described. Here, the number m of transmission antennas is 2 and the number n of reception antennas is 2. In a transmission device 5, the same spreading code is used for all transmission antennas 21-1 and 21-2. Additionally, a delay time in the path of a reception antenna 11-1 is identical to that of a reception antenna 11-2. The propagation path for each antenna includes two paths, as shown in FIG. 2. Furthermore, a reception device 4 has one de-spreader unit.
FIG. 3 illustrates an exemplary configuration of the reception device 4. Reception signals r1(t) and r2(t) received by the respective first antenna 11-1 and the second antenna 11-2 are expressed by the following equations:
                    r        1            ⁡              (        t        )              =                            h                      11            -            1                          ⁢                  c          ⁡                      (                          t              -                              τ                1                                      )                          ⁢                              b            1                    ⁡                      (                          t              -                              τ                1                                      )                              +                        h                      11            -            2                          ⁢                  c          ⁡                      (                          t              -                              τ                2                                      )                          ⁢                              b            1                    ⁡                      (                          t              -                              τ                2                                      )                              +                        h                      12            -            1                          ⁢                  c          ⁡                      (                          t              -                              τ                1                                      )                          ⁢                              b            2                    ⁡                      (                          t              -                              τ                1                                      )                              +                        h                      12            -            2                          ⁢                  c          ⁡                      (                          t              -                              τ                2                                      )                          ⁢                              b            2                    ⁡                      (                          t              -                              τ                2                                      )                              +                        n          1                ⁡                  (          t          )                                        r        2            ⁡              (        t        )              =                            h                      21            -            1                          ⁢                  c          ⁡                      (                          t              -                              τ                1                                      )                          ⁢                              b            1                    ⁡                      (                          t              -                              τ                1                                      )                              +                        h                      21            -            2                          ⁢                  c          ⁡                      (                          t              -                              τ                2                                      )                          ⁢                              b            1                    ⁡                      (                          t              -                              τ                2                                      )                              +                        h                      22            -            1                          ⁢                  c          ⁡                      (                          t              -                              τ                1                                      )                          ⁢                              b            2                    ⁡                      (                          t              -                              τ                1                                      )                              +                        h                      22            -            2                          ⁢                  c          ⁡                      (                          t              -                              τ                2                                      )                          ⁢                              b            2                    ⁡                      (                          t              -                              τ                2                                      )                              +                        n          2                ⁡                  (          t          )                    where the fluctuation of the propagation path is sufficiently delayed with respect to the fluctuation of the transmission signal. τ1 and τ2 represent paths. c(t−τ1), bm(t−τ1), nn(t) are a spreading code, a data signal from an m-th transmission antenna 21-m, and a noise signal of an n-th reception antenna 11-n, respectively. hnm-1 represents a first path between the m-th transmission antenna 21-m and the n-th reception antenna 11-n. As described above, m=n=2.
A channel estimation unit 12 carries out channel estimation using received signals and outputs the obtained channel estimation value H. In general, a spread-spectrum communication system selects the paths in descending order of power. Accordingly, a maximum-power-path selection unit 41 selects a path having the largest power and outputs a maximum path signal Xmax. Here, it is assumed that a preceding path has larger power. In this case, de-spreader units 14-1 and 14-2 de-spread based on the maximum path signal in synchronization with the timing of the preceding path.
For the sake of simplicity, one received symbol is focused on. A de-spreading signal y1 of the first reception antenna 11-1 is given by the following equation (1):
                                                                        y                1                            =                                                ∫                                      τ                    2                                                  ⁢                                                      c                    ⁡                                          (                                              t                        -                                                  τ                          1                                                                    )                                                        ⁢                                                            r                      1                                        ⁡                                          (                      t                      )                                                        ⁢                                                                          ⁢                                      ⅆ                    t                                                                                                                          =                                                                    h                                          11                      -                      1                                                        ⁢                                      b                    1                                                  +                                                      h                                          12                      -                      1                                                        ⁢                                      b                    2                                                  +                                  R                  ⁢                                      (                                                                  τ                        1                                            -                                              τ                        2                                                              )                                    ⁢                                                            ∑                                              m                        =                        1                                            2                                        ⁢                                                                  h                                                                              1                            ⁢                            m                                                    -                          2                                                                    ⁢                                                                        b                          m                                                ⁡                                                  (                                                      t                            -                                                          τ                              2                                                                                )                                                                                                                    +                                  n                  1                  ′                                                                                        (        1        )            where Ts is one symbol time, R is an auto-correlation function of a spreading code, and n′1 is Gaussian noise.
As shown in FIG. 4, a widely used auto-correlation function of a spreading code exhibits a sharp peak at a time difference of zero and exhibits a sufficiently small value at a time difference of other values. Therefore, the second term of equation (1) can be ignored here.
As a result, the de-spreading signal y1 is expressed as:y1=h11-1b1+h12-1b2+n1.
Similarly, a de-spreading signal y2 of the second reception antenna 11-2 is given by the following equation:y2=h21-1b1+h22-1b2+n2.
A demodulation unit 15 inputs the de-spreading signals and carries out demodulation to reproduce a data sequence. The de-spreading signal is given by the following equation (2) in vector representation:
                                                        Y              =                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                            ]                                                                                        =                                                                    [                                                                                                                        h                                                          11                              -                              1                                                                                                                                                            h                                                          12                              -                              1                                                                                                                                                                                                        h                                                          21                              -                              1                                                                                                                                                            h                                                          22                              -                              1                                                                                                                                            ]                                    ⁡                                      [                                                                                                                        b                            1                                                                                                                                                                            b                            2                                                                                                                ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                      ]                                +                Hb                +                n                                                                        (        2        )            
The demodulation unit 15 reproduces a transmission sequence by, for example, premultiplying the equation by a reverse matrix of the channel estimation value H, as follows:b0=H−1Y=b+H−1n. 
When all of the propagation paths are independent, a reverse matrix of the above-described channel matrix can be correctly obtained. However, if channel replies have correlation, a reverse matrix does not exist, thereby disabling reception.
For example, if a propagation path between the first transmission antenna 21-1 and the first reception antenna 11-1 has complete correlation with a propagation path between the first transmission antenna 21-1 and the second reception antenna 11-2 and if a propagation path between the second transmission antenna 21-2 and the first reception antenna 11-1 has complete correlation with a propagation path between the second transmission antenna and the second reception antenna, h11-1=h21-1, h11-2=h21-2, h12-1=h22-1, and h12-2=h22-2. In this case, the channel estimation value H is given by the following equation (3):
                    H        =                  [                                                                      h                                      11                    -                    1                                                                                                h                                      22                    -                    1                                                                                                                        h                                      11                    -                    1                                                                                                h                                      22                    -                    1                                                                                ]                                    (        3        )            
This results in |H|=0, and therefore, the inverse matrix does not exist. Consequently, it is hard to perform the demodulation process.
Accordingly, it is an object of the present invention to solve the above-described problem and provide a reception device that achieves superior communication quality even when propagation paths have correlation.
It is another object of the present invention to provide a radio communication system using the reception device.